Recent demands for solar cells are directed to higher efficiency and lower costs at the same time. In this context, the OECO (obliquely evaporated contact) process attracts public spotlight. The OECO process is a method of fabricating solar cells proposed by R. Hezel et al. of Institut für Solar energie for schung GmbH Hameln/Emmerthal (ISFH), Germany, and is representatively disclosed in Renewable Energy, Vol. 14, p. 83 (1998) (a solar cell fabricated by the OECO process may occasionally be referred to as OECO solar cell, hereinafter). A method of fabricating the OECO cell will briefly be explained referring to FIG. 12. The OECO solar cell is configured so that a plurality parallel grooves 2′ (having a sectional form of rectangular, semicircle, triangular or the like) are carved on the main surface, which later serves as a light-receiving surface, of a silicon single crystal substrate 24′ (referred to as “semiconductor single crystal substrate”, or more simply to as “substrate”, hereinafter), and so that each groove 2′ has an electrode 6′ for extracting output on the inner side face 2′a on one side as viewed along the width-wise direction. The grooves 2′ are generally carved using a dicing saw. More specifically, the silicon single crystal substrate 24′ is placed on the surface of a working table, and the grooves 2′ are carved on the groove forming surface (light-receiving surface) 1′a of the silicon single crystal substrate 24′ by horizontally moving the dicing saw while keeping a constant distance away from the surface of the working table. The formation of the electrode 6′ on one side face 2′a of each groove 2′ is conducted by vacuum evaporation which proceeds obliquely with respect to the main surface of the semiconductor single crystal substrate 24′, which later serves as the light-receiving surface 1′a. This process allows an electrode material to deposit selectively on the side faces 2′a of the grooves and the top surface 2′b of projected ridges based on the shadowing effect exerted by the grooves itself on the metal to be deposited. Thickness of thus deposited metal differs between areas on the top surface 2′b of the projected ridges and areas on the inner side faces 2′a, so that an etching process successive to the evaporation process can successfully remove the metal so as to leave the electrodes only on the side faces, to a thickness as much as equivalent to a difference between the thickness on the side face 2′a and thickness on the top surface 2′b. 
This constitution successfully reduced the shadowing loss of the solar cell to as small as approximately 5% of the total light-receiving area. Because a typical solar cell having the electrodes formed by the screen printing method generally suffers from a shadowing loss of as large as approximately 12%, it is understood that the OECO solar cell has a sharply reduced shadowing loss, and that a large energy conversion efficiency is attainable.
Solar cells currently put into practical use can be classified by source materials into those of silicon-base (single crystal, poly-crystalline and amorphous), those of compound semiconductor-base, and others. Among others, solar cells using silicon single crystal substrate are most widely fabricated by virtue of their excellent energy conversion efficiency and low production cost. The silicon single crystal substrates used for solar cells are generally fabricated by slicing a single crystal ingot using a wire saw, where the ingot is obtained by the Czochralski method (simply referred to as “CZ method”, hereinafter), or the floating zone method (simply referred to as the FZ method, hereinafter). The substrates sliced out by the dicing saw are used as-sliced.
The slicing using the wire saw is, however, disadvantageous in that the amount of an abrasive grain retained in the working site increases as the depth of cutting into the ingot increases, and this gradually increases the cutting width. As a consequence, the semiconductor single crystal substrate 24′ will have a thickness such as decreasing from the start-of-cutting side (left hand side in the drawing) towards the end-of-cutting side (right hand side in the drawing). It is thus understood that use of the as-sliced semiconductor single crystal substrate 24′ is advantageous in reducing the production costs to a considerable degree through shortening of the process time, but is disadvantageous in making the thickness of the semiconductor single crystal substrate 24′ non-uniform. The non-uniformity of the thickness amounts as much as 20 to 30 μm for 4-inch-diameter substrate, and the non-uniformity becomes larger as the diameter of the substrate increases.
Any conventional method of carving the grooves on the substrate 24′ for fabricating the solar cells will be unsuccessful in obtaining a constant depth of the grooves 2′ as measured from the light-receiving surface 1′a over the entire surface because of the non-uniformity in the thickness of the substrate 24′. More specifically, cutting with an upper-edge-type dicing saw, which keeps the lower edge thereof at a constant level of height, will carve deeper grooves 2′ in the thicker portion of the substrate 24′.
On the substrate 24′ having the grooves of non-uniform depths, formation of the electrodes 6′ on the side faces 2′a of the grooves by the above-described vapor deposition process will result in heights of the electrode 6′ smaller than the designed values, or undesirable deposition of the metal also on the bottom surface 2′c of the grooves. The electrode 6′ having a height smaller than a designed value will cause a larger conduction loss at the electrode 6′. The formation of the metal on the bottom surface 2′c of the groove will increase the shadowing loss and thus ruin the energy conversion efficiency. Etching for removing the excessive deposition of the metal will, however, decrease electrode forming area and will thus increase the resistance loss. Both increases in the shadowing loss and resistance loss result in lowering of the energy conversion efficiency of the solar cell as a natural consequence.
Besides the above-described OECO solar cells, also other types of solar cells may cause variation in the characteristics if the depths of the grooves carved in the substrate 24′ are non-uniform. In an exemplary case where the depth of the groove for electrode contact carved on the back surface of the solar cell is larger than a designed value, the boundary area between the electrode and substrate increases, and this results in increase in recombination rate at the boundary. On the contrary, the depth smaller than the designed value result in poor contact between the electrode and substrate, and this increases the contact resistance. In a still another exemplary case where the depths of the grooves carved on the light-receiving surface of the solar cell are non-uniform, variation in the boundary area causes difference in the recombination rates in the thickness-wise direction and the in-plane direction normal thereto. These variations in the characteristics result in variation in the output voltage, and may eventually lower the output of the solar cell.
It is therefore a first object of the invention to provide a solar cell having adequately-formed electrodes based on the OECO process in order to surely suppress the shadowing loss and resistance loss. It is a second object of the invention to provide a method of fabricating a solar cell having the grooves formed therein such as OECO solar cells, capable of readily making the depth of grooves uniform, and of realizing higher efficiency of the solar cell at low cost.